[FEAT] Implement plugable authentication for OTLP HTTP

[marcalff]

Many systems expose an OTLP HTTP endpoint.

Currently, the only auth supported by opentelemetry-cpp in the OTLP HTTP exporter, is to provide an http header in plain text, with magic key/value pairs.

opentelemetry-cpp needs to be expanded to:

• define an OTLP HTTP authentication interface
• allow the OTLP HTTP exporter to use an arbitrary authentication
• sign HTTP requests using the authentication implementation provided.

When an application wants to talk to an endpoint using the XYZ authentication method, the application is then responsible to provide an authentication plugin that implements the XYZ method.

Authentication plugins are a natural extension point for file configuration, see related:

• Authenticator PoC opentelemetry-configuration#275